1. Field of the Invention
The invention relates to a process for the common (simultaneous) separation of two or more contaminating elements from electrolyte solutions (usually a mineral acid such as sulfuric) of valuable metals by solvent extraction from the liquid phase and subsequent recovery of the contaminating elements for reutilization.
2. Statement of Related Art
Here as well as in the subsequent description and in the claims "valuable metals" are understood to mean those desirable metallic elements which are recovered from their natural sources, more particularly from their ores, by way of industrial processes and are put into use in the metallic state, if desired as alloys with other metals, excluding metals referred to herein as contaminants. In the recovery of valuable metals hydrometallurgical processes play an important role in addition to pyrometallurgical processes. Frequently the metals or metal salts contained in the ores are developed or leached with aqueous systems, and the valuable metal is recovered from such metal salt solutions by electrolysis. However, the efficiency of the electrolysis of such aqueous solutions is highly impaired by the fact that most of the valuable metals are associated with other metals in the ores. Thus, the electrolyte solutions for recovering a valuable metal nearly always contain larger or lesser amounts of contaminating elements which adversely affect the electrolytic isolation of the valuable metal or are deposited as undesired impurities together with the valuable metal. Therefore, in order to increase the purity of the electrolytically deposited valuable metals, it is desired to remove as much as possible of all contaminating elements from the valuable metal electrolyte solutions.
Isolation of the metals copper, zinc, cobalt or nickel is possible by means of electrolysis. However, aqueous solutions obtained from leaching ores containing such metals usually contain larger or lesser amounts of contaminating elements. Satisfactory processes for removing--and if possible recovering--such contaminating elements are in demand not only because thereby the quality and the quantity of the deposited valuable metals will be improved, but also because a recovery and recycling of the contaminating elements makes economic and ecological sense.
The recovery of superpure copper by pyrometallurgical refining, for example, is characterized by two process steps proceeding separately from each other. In the first step, melt metallurgical refining, relatively impure crude copper originating from smelting copper ores is precipitated from the melt ("anode furnace"). In the subsequent refining electrolysis contaminating elements are removed and in part deposited in the anode slime, while highly conductive "electrolytic copper" (up to 99.99% pure) is deposited on the cathode. The cathode blocks comprising superpure copper thus prepared may then be further processed by plastic deformation (rolling, drawing, pressing etc.).
The elements arsenic, antimony, bismuth and iron are among numerous interfering factors of the copper-refining electrolysis which are of particular relevance in the electrolyte solutions, the amounts of which increase in the course of the process. Such contaminating elements are accumulated particularly fast in strongly acidic sulfuric acid solutions, if crude ores rich in such contaminating elements are smelted and processed in the subsequent refining procedures, as is done to a growing degree. As a consequence, it also must be expected that higher concentrations of the contaminating elements arsenic, antimony, bismuth and iron occur in the copper-refining electrolyte solutions. An accumulation of the contaminating elements not only deteriorates the quality of cathodically deposited copper which will contain increasing amounts of arsenic, antimony and bismuth impurities, but also reduces the current efficiency (due to the potential jump of Fe.sup.2+ to Fe.sup.3+) and, thus, increases the energy expenditure for the process.
Numerous processes are known from the prior art wherein arsenic, and in a few cases also antimony, can be removed from the solutions used for the deposition. The common feature of all these processes is that as soon as a critical concentration of the contaminating elements in the electrolyte solutions is reached, more specifically at a limit of 10 g/l of arsenic, a fractional stream of the electrolyte solution is withdrawn and then subjected to a "copper-recovery electrolysis". Thereby not only the residual copper is electrolytically deposited from the solutions, but also the aforementioned contaminating elements are removed therefrom (in an ultimate stage, "liberator cells"). The relatively impure copper obtained thereby must be once more remelted and brought to desired purity prior to use. After the deposition of these elements there remain relatively high amounts of nickel in the strongly acidic sulfuric acid solutions, which upon evaporation are precipitated as crude nickel sulfate and subjected to further purification for removing contaminations by iron, arsenic and, if desired or required, antimony. The resulting concentrated waste sulfuric acid is mostly recycled into the process.
Canadian patent 1,070,504 (and corresponding German patent document 26 03 874) describes a process for removing arsenic contaminant from copper refining electrolytes wherein the aqueous electrolyte solution is contacted with an organic phase containing tributylphosphate (TBP) to extract the arsenic contained in the solution into the organic phase. This organic phase is then brought into contact with water or an aqueous alkaline solution so as to transfer the arsenic into the aqueous phase from which it is subsequently extracted.
In a process according to U.S. Pat. No. 4,115,512 an organic phase is used as extractant which contains tributylphosphate in admixture with quaternary ammonium compounds.
Tributylphosphate as well as esters of phosphonic acid, phosphonous acid, phosphinic acid and phosphinous acid are used together with organic solvents as extractants in processes according to U.S. Pat. No. 4,102,976 (and corresponding German patent document 26 14 341) and U.S. Pat. No. 4,061,564 (and corresponding German patent document 26 15 638), for removing arsenic or antimony from copper electrolyte solutions.
Arsenic is removed from electrolytes of copper refining by means of a process according to U.S. Pat. No. 4,503,015 (and corresponding European patent document 106,118) which employs organophosphorus compounds, for example trioctylphosphine oxide (TOPO), in organic solvents such as kerosene.
U.S. Pat. No. 4,547,346 (and corresponding German patent document 34 23 713) discloses another process for removing arsenic from an acidic sulfuric acid-containing copper electrolyte, wherein C.sub.5-13 aliphatic alcohols, and preferably 2-ethyl-1-hexanol, in an organic phase are used as extractants. A large proportion of arsenic, though not the total amount thereof, is removed from the electrolyte solution in the course of six extraction cycles.
However, all of the above prior art process have various drawbacks. The reagents must be employed at high concentrations in order to accomplish an efficient extraction of the contaminating elements from the electrolyte solutions. This seems clearly apparent, for example, from German patent document 26 15 638 (claim 4 in combination with page 4, penultimate paragraph, of the description), which is equivalent to U.S. Pat. No. 4,061,564. Moreover, most of the processes require a high concentration of acid in the extraction solutions which is virtually effected by concentrating the electrolyte to increase the sulfuric acid concentration thereof from an initial 100 to 250 g/l up to about 500 g/l. At those high sulfuric acid concentrations, the organo-phosphorus compounds not only extract the contaminating elements from the solutions, but also deliver considerable amounts of sulfuric acid into the organic phase. Consequently, it is then necessary to provide several stages of washing whereby the extracted sulfuric acid is recovered and recirculated into the process. Furthermore, the organophosphorus extractants (and more particularly TBP) are not sufficiently stable at those high acid strengths, so that their efficiency is reduced. Additionally, in all of these proceses as a supplement to improve separation of the organic phase from the inorganic phase, a modifier, mostly isodecanol, must be added to the extractant, which modifier under certain circumstances may even accelerate the decomposition of the extractant.
In addition, all the prior art processes have drawbacks related to the reextraction of the contaminating elements from the organic phase following the actual extraction procedure. Thus, according to U.S. Pat. Nos. 4,061,564 and 4,102,976 arsenic is separated from the organic phase by means of aqueous alkali solutions. However, arsenic is thereby obtained with the oxidation numbers (III) and (V). In order to obtain the desired As.sub.2 O.sub.3 as the final product, pentavalent arsenic has to be additionally reduced prior to or during the reextraction, conventionally by the use of SO.sub.2. Therefor, a further process step employing additional equipment and chemicals is needed.
According to U.S. Pat. No. 4,503,015 hydrochloric acid and other aqueous mineral acids are used for reextracting arsenic. It is only by strictly controlling the chloride content in the reextraction that Cl.sup.- can be prevented from getting into the refining electrolyte and thereby undesirably deteriorating the copper refining procedure. To this end, expensive multistage cycle systems are required in practice.
The difficulties in the removal of contaminating elements from copper electrolyte solutions are encountered in a similar manner in the occurrence of contaminating elements in aqueous electrolyte solutions of other valuable metals, such as zinc or nickel. In individual cases emphasis may be laid on the removal of one definite contaminating element or on the removal of a group of such elements.
In view of the state of prior art as described there has been a great demand for a process for the common (simultaneous) separation of two or more contaminating elements from electrolyte solutions of valuable metals and the subsequent recovery of the contaminating elements for further utilization. Such process must be capable of removing at least two, preferably all of the four contaminating elements arsenic (As), antimony (Sb), bismuth (Bi) and iron (Fe) and, in addition, use simple extractants which are readily available. These extractants, moreover, should be stable under the strongly acidic conditions and conventional process temperatures encountered in the electrolyte solutions of valuable metals.
There has long been sought a solution to the problems and the disadvantages inherent to the prior art processes for removing arsenic from copper-refining solutions. Ideally, the solution will be able to remove, and to recover, the contaminating elements from the organic extract phase and thereby obtain a sulfuric acid solution free from contaminating elements which may be recirculated to the refining electrolyte without causing any problem. In such a process it should be ensured that the efficiency is not reduced by contamination of the electrolyte by impurities entrained from the preceding extraction and/or reextraction steps.
Processes for the selective extraction of individual metal ions from aqueous solutions by means of a hydroxamic acid dissolved in an organic solvent are known in the art.
In U.S. Pat. No. 3,900,551 (and corresponding German patent document 22 10 106) transition metals from partially radioactive solutions of reprocessing plants are extracted with hydroxamic acid having the general formula (A) ##STR1## wherein the moieties R', R" and R"' all are alkyl. The pH is important, and the value for the aqueous recovery solution is 2 or more pH units less than the minimum pH of the organic phase.
According to U.S. Pat. No. 3,464,784, vanadium is extracted from aqueous solutions containing tetravalent vanadium by means of organosoluble hydroxamic acids having the general formula (B) ##STR2## wherein R is a C.sub.7-44 alkyl, cycloalkyl or aryl, neo-alkyl moieties being preferred which contain a quaternary carbon atom adjacent to the carbonyl.
In "J. Chem. Research" (S) 1982, pp. 90, et seq., the solvent extraction of transition metals with versato-hydroxamic acids of above general formula (B) has been described wherein the moiety R is a branched C.sub.10-15 alkyl.
The solvent extraction of various metal isotopes from aqueous solutions of reprocessing plants for radioactive residual materials by means of trialkylacetylhydroxamic acid is described in "Reprints of the ISEC '86, Sep. 11-16", Munich, pp. 355-362.
However, the common (simultaneous) removal of at least two preferably three, most preferably all of arsenic, antimony, bismuth or iron from copper electrolyte solutions has not been disclosed nor contemplated in any of the above quoted literature or patent references.